A variety of modern electronics utilize load switches to control the delivery of power to loads that draw current from the available power supply, such as a motor or SMPS. Load driver circuits are used to exert control over a load switch. In addition to determining when the load switch provides power to a load, a load driver circuit may also control the rate at which properties of the load are changed. The rate at which these properties change is often referred to as a slew rate. Load switches are commonly implemented using MOSFETs, which provide precise control over the delivery of power to a load and over slew rates, such as the change in voltage on a phase node of a motor or the voltage drop on the inductance of an SMPS.
Once a determination has been made to switch power to a load, a load driver circuit is typically configured to power the load as quickly as possible. This maximizes efficiency in terms of minimizing the latency in powering the load, such that the component being powered can perform its intended function. However, practical limits exist on how quickly a load can be powered by a load driver circuit.
Many types of current loads (for instance, electric motors) are sources of impedance. The impedance of these loads cause problematic side effects that result from powering them. For example, in the case of a load that is a motor, the load current path is switched between being driven by a high-side and low-side driver, which results in the current path switching between supply and ground paths. This switching causes the voltage on the load to change quickly, which results in a kickback charge flowing back to the load switch. This kickback voltage can traverse the load switch, and can result in the unintended switching of the opposite load switch. This, in turn, has consequences ranging from reduced efficiency in benign cases to damage to the load driver circuit and/or the load switch.
In addition to kickback, powering a current load can result in electromagnetic interference (EMI) being generated. One particularly relevant source of EMI is the electromagnetic force that results from rapidly powering an inductive load, such as a motor or from the rapidly changing current in the supply and ground wires. The greater the rate of change of voltage powering the load, the greater the magnitude of the induced magnetic field, and the greater the levels of resulting EMI. Even modest amounts of EMI can result in spurious currents in the system that can cause malfunctions in neighboring circuitry and potentially even damage neighboring circuits.
Fast current changes through the switches will generate large voltage spikes due to the parasitic inductance on the current path. These large spikes can exceed switch and driver circuit safe operating limits and damage parts.
In general, these problems caused by the application of power to a load can be ameliorated by slowing the rate at which the voltage on the load changes. Moderating the rate at which voltage on the load changes results in a decrease in kickback and the generation of EMI.
Changing the voltage on a load more slowly can at least partially alleviate some of these problems, but it introduces an undesirable inefficiency into the system. By delaying the time required to reach the supply voltage (or ground, depending on whether the load is being switched on or off), this introduces a latency in the response time of the load. Any such delays accumulate over time and cause unacceptable inefficiencies that ripple throughout the system. Thus, it is desirable to apply a voltage to a load, such as a motor, at a rate that minimizes the latency in the response time for the load, yet does not produce undesirable levels of kickback current and EMI.